Acrylic-based latex paints employ acrylic latex binders as film formers and binders for pigments, fillers and the like, which are used in latex paints. The latex binders typically comprise acrylic emulsion polymers. The properties that are desirable in acrylic latex binders for use in coating compositions are uniform particle size, mechanical stability, especially at high shear, and the absence of coagulum. Additionally, the latex should dry to a clear, glossy and water resistant film. The properties that are desirable in an acrylic latex-based coating compositions are freeze thaw stability, gloss, abrasion resistance, block resistance, controlled drying time and proper rheological properties.
Traditionally, emulsifying agents such as anionic surfactants are used to control the latex particle size. Also, nonionic emulsifiers such as nonyl phenol surfactants are used to stabilize the latex to give shear stability and freeze-thaw stability. With respect to acrylic latexes, the standard stabilizers are alkyl phenol ethoxylate (APE) surfactants. As APE surfactants and their degradation products are suspected as being environmentally unfriendly, there is a growing trend to find alternative stabilizers which are environmentally friendly and which provide all of the properties required for use in acrylic latex binders and paints.
Extensive studies have been conducted on the use of linear alcohol and fatty acids as replacements for APE surfactants. Some of the linear alcohols have been found to work as stabilizers, but the performance generally has not been as good as that of the APE. Accordingly, there is a need for new class of stabilizers for use in preparing acrylic latex binders.
The use of nonpolymerizable carbohydrate-based amphiphiles to stabilize latexes of polystyrene has been reported, Emulsion Polymerization of Styrene in the Presence of Carbohydrate-based Amphiphiles, Demharter et al Polymer Bulletin 34, 691-696 (1995). As noted therein, N-alkyl and N-alkyl-N'-methyl semicarbazones of maltose and maltotriose were examined as non-ionic emulsifiers in the preparation of polystyrene via emulsion polymerization in order to understand the influence of alkyl chain length and amphiphile concentrations on critical micelle concentration, latex particle sizes, particle size distributions, and rheology. The report fails to address on any level the issue of whether or not the amphiphiles would be useful to stabilize other types of emulsion polymerization, e.g., preparation of acrylic latex via emulsion polymerization, or that latexes prepared with the amphiphiles would be able to provide freeze-thaw stability and other required paint properties, such as abrasion resistance and gloss, to latex acrylic paint compositions which comprised the latexes stabilized with the amphiphiles.
It now has been found that certain nonpolymerizable, saccharide-derivatives are useful as emulsion stabilizers during the preparation of acrylic-based latex binders via emulsion polymerization. The binders are environmentally friendly as opposed to APE surfactants, in that the saccharide materials are derived from renewable resources rather than from petrochemicals, as are ethoxylated materials, and ultimately are biodegradable. The latex binders are capable of providing both freeze-thaw stability and sufficient or improved properties of gloss and abrasion resistance to acrylic latex-based paints employing the acrylic latex binders.